The invention relates generally to processes for forming P-N junctions in semiconductor devices, and to the resultant devices. The invention relates more particularly to a process for forming a passivated P-N junction in a mesa structure. The passivation layer is formed by growing silicon dioxide on the mesa sidewall. This invention relates more specifically to a method to compensate for the curvature in the P-N junction which occurs during the oxidation and thus keep the P-N junction relatively plane.
U.S. Pat. No. 4,740,477 to Einthoven et al issued Apr. 26, 1988 discloses a semiconductor device having a P.sup.+ /N.sup.- /N.sup.+ junction in a mesa structure, which patent is hereby incorporated by reference as part of the present disclosure. The mesa can be formed by an anisotropic etch, and the walls of the mesa slope outwardly from a top surface thereof toward the N.sup.- /P.sup.+ junction. After the mesa is etched, the highly concentrated N.sup.+ region is diffused more deeply into the mesa but not below the base of the mesa, and the diffusion adjacent to the sidewall of the mesa takes on a generally concave shape relative to the N.sup.- layer as it penetrates into the N.sup.- layer. The result is that the thickness of the lightly doped N.sup.- region adjacent to the sidewall of the mesa is greater than the thickness of the lightly doped region at the interior regions. Consequently, electric fields adjacent to the sidewall of the mesa are reduced and the tendency for surface breakdown is reduced. The mesa also includes a grown silicon dioxide layer surrounding its sidewall to passivate the device.
While the structure of U.S. Pat. No. 4,740,477 has proven highly effective and useful, in some applications, particularly those requiring a low cost junction, it is desirable to provide a diffused P-N junction with bulk breakdown without the intervening lightly doped region.
Attempts have been made to passivate a diffused P-N junction with a grown oxide layer to prevent parasitic breakdown. However, the resultant breakdown occurred adjacent to the oxide layer because the oxidizing step curves the portion of the P-N junction adjacent to the oxide layer toward the P layer.
A general object of the present invention is to provide a process for making a P-N junction with a greater breakdown voltage adjacent to the oxide layer than exhibited by the aforesaid diffused P-N junction portion, for a given bulk breakdown.
Another general object of the present invention is to provide a P-N junction of the foregoing type in a mesa structure which structure is passivated with a grown silicon dioxide layer around its sidewall.
Another general object of the present invention is to provide semiconductor devices of the foregoing types which are useful as zener and other types of diodes that in essence exhibit avalanche breakdown in the bulk region.
Another general object of the present invention is to compensate the curving of the diffused P-N junction portion adjacent to the oxide layer by subsequent diffusions and oxidations which curve the P-N junction in opposite directions.